2b:null 2c:["$","title",null,{"children":"ods-color-gradient algorithm explained | Frédéric Passaniti | Observable"}] 31:[] 2f:{"id":"f02e8dd4f5d90914","slug":"ods-color-gradient-algorithm-explained","title":"ods-color-gradient algorithm explained","thumbnail":"380d4edbc43289d110b83c357795a0d60f8b007072b0f945fb6fe9e2f5e14f0d","access_level":"public","create_time":"$D2020-07-21T10:02:27.687Z","update_time":"$D2021-07-30T13:54:34.726Z","view_time":null,"first_public_time":"$D2020-07-21T10:02:32.419Z","fork":{"id":"ab78a58dfeb73cee","slug":"untitled","title":"ods-color-gradient algorithm explained","owner":{"id":"190d062db242c561","login":"fpassaniti","name":"Frédéric Passaniti"}},"stars":0,"starred":false,"owner":{"type":"team","id":"190d062db242c561","login":"fpassaniti","name":"Frédéric Passaniti","avatar_url":"https://avatars-next.observableusercontent.com/54707367e49ff48f413a075bace8b2dc7605877cbe2acbf85fe7aecbcaef60a8","tier":"starter_2024"},"version":1643,"publish_version":1643,"role":null,"authored":false,"authors":[{"type":"individual","id":"a20140efc14d7638","login":"fpassaniti","name":"Frédéric Passaniti","avatar_url":"https://avatars-next.observableusercontent.com/54707367e49ff48f413a075bace8b2dc7605877cbe2acbf85fe7aecbcaef60a8","tier":null}],"members":[{"type":"individual","id":"a20140efc14d7638","login":"fpassaniti","name":"Frédéric Passaniti","avatar_url":"https://avatars-next.observableusercontent.com/54707367e49ff48f413a075bace8b2dc7605877cbe2acbf85fe7aecbcaef60a8","tier":null,"role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"subscribed":null,"forks":0,"collection":null,"type":"notebook","body":{"title":"ods-color-gradient algorithm explained","cells":[{"id":214,"value":"scale_pow = md`# ods-color-gradient algorithm explained\n\n[d3.scalePow](https://github.com/d3/d3-scale#power-scales) creates a power based scale. The transform applied to values is of the type ${tex`y = x^k`} where *k*, the exponent, can be any real number.\n\nFor ${tex`k=0.3`}, ${tex`x∈[0:1]`} : it approximates log scale.\n\nFor ${tex`k=1`} : it is a linear scale.\n`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1535,"value":"md`x axis is the input value from 0 to 1, y the result that will be used to get the corresponding color`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":229,"value":"visualizeScale(\n d3\n .scalePow()\n .exponent(k)\n .domain([0, 1])\n)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":231,"value":"viewof k = slider({ min: 0, max: 10, value: 1, description: \"exponent k\" })","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1312,"value":"md`# No odsColorGradientNbClasses defined\n\nWithout classes, it simply apply the pow scale to input data.\n\nFor x within 0 and 1, ${tex`x^k`} and get the corresponding color within rgbLowValue - rgbHighValue range`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":227,"value":"md`Select your color boundaries and see how the color scale change by dragin the exponent slider`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1541,"value":"md`### RGB for lower value`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1194,"value":"viewof rgbLow = colorPicker(\"rgb(180, 197, 241)\")","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1543,"value":"md`### RGB for higher value`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1197,"value":"viewof rgbHigh = colorPicker(\"rgb(20, 33, 96)\")","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1310,"value":"html`

\n${d3.ticks(0, 1, 200)\n .map(d3.scalePow().exponent(k).domain([0,1]).range([rgbLow,rgbHigh]))\n .map(t => ` `)}

`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1315,"value":"md`# odsColorGradientNbClasses`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1231,"value":"md`### Select the number of desired classes/colors/steps`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1224,"value":"viewof nbClasses = {\n const form = html``;\n form.i.oninput = () => (form.value = form.i.valueAsNumber);\n form.i.oninput();\n return form;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1258,"value":"md`Setting the number of classes will split the x axis by nbClasses.\nEach range boundaries are powered by the selected exponent. Change the number of classes and see the result`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1574,"value":"md`### ${tex`x`} axis divided by nbClasses`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1571,"value":"visualizeTicksValues(d3.scaleLinear().domain([0,1]).range([0,1]), \n new Array(nbClasses + 1).fill(undefined).map((x, i) => i * 1 / nbClasses))","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1578,"value":"md`### ${tex`y = x^k`} axis`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1329,"value":"visualizeTicksValues(d3.scalePow().exponent(k).domain([0,1]).range([0,1]), \n new Array(nbClasses + 1).fill(undefined).map((x, i) => i * 1 / nbClasses))","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1586,"value":"md`### Finally to define the color thresholds, we get the corresponding ${tex`x`} values to match the ${tex`y`} axis. There is nbClasses - 1 threshold that defines the classes/groups of values to be applied to the input data`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1466,"value":"visualizeTicksValues(d3.scaleLinear().domain([0,1]).range([0,1]), \n new Array(nbClasses + 1).fill(undefined).map((x, i) => i * 1 / nbClasses).map(d3.scalePow().exponent(k).domain([0,1]).range([0,1]).invert))","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":285,"value":"html`

\n${d3.ticks(0, 1, 200)\n .map(d3.scaleThreshold()\n.domain(new Array(nbClasses + 1).fill(undefined).map((x, i) => i * 1 / nbClasses).slice(1, nbClasses).map(d3.scalePow().exponent(k).domain([0,1]).range([0,1]).invert))\n.range(new Array(nbClasses + 1).fill(undefined).map((x, i) => i * 1 / nbClasses).slice(0, nbClasses).map(d3.scaleLinear().domain([0,1]).range([rgbLow,rgbHigh]))))\n .map(t => ` `)}

`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1624,"value":"md`## NOTAs\n- Without classes, it's a simple linear or pow color gradient\n- The more classes you have, the more it looks like a color gradient\n- nbClasses parameter is mandatory to use ods-legend widget with display=steps, as the widget relies entirely on color classes to render`\n","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1207,"value":"md`# APPENDIX\n## Technical imports, do not read`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1187,"value":"d3 = require(\"d3@5\")","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1189,"value":"function visualizeScale(scale) {\n const w = Math.min(600, width),\n h = 300,\n m = width > 599 ? 30 : 10;\n const x = d3.scaleLinear()\n .domain(d3.extent(scale.domain()))\n .nice()\n .range([m, w - m]),\n y = d3.scaleLinear()\n .domain(d3.extent(scale.range()))\n .nice()\n .range([h - m, m]),\n svg = d3.create(\"svg\")\n .attr(\"width\", width + 20)\n .attr(\"height\", h + 20),\n g = svg.append(\"g\"),\n line = [];\n\n for (let i = m + 1e-6; i < w - m; i += 1) {\n const X = x.invert(i),\n Y = scale(X),\n j = y(Y);\n line.push([i, j]);\n }\n\n g.append(\"path\")\n .attr(\"d\", \"M\" + line.join(\"L\"))\n .style(\"stroke\", \"black\")\n .style(\"fill\", \"none\");\n\n g.append(\"g\")\n .attr(\"transform\", `translate(${m},0)`)\n .call(d3.axisLeft(y));\n g.append(\"g\")\n .attr(\"transform\", `translate(0,${y(0)})`)\n .call(d3.axisBottom(x));\n\n return svg.node();\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":642,"value":"function visualizeTicks(scale, tickArguments) {\n const height = 20, m = width > 599 ? 90 : 10;\n\n if (tickArguments === undefined) tickArguments = [];\n\n scale.range([m, width - m]);\n\n const svg = d3.create(\"svg\")\n .attr(\"width\", width)\n .attr(\"height\", height);\n\n svg.append(\"g\").call(d3.axisBottom(scale).ticks(...tickArguments));\n\n return svg.node();\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1383,"value":"function visualizeTicksValues(scale, tickValues) {\n const height = 20, m = width > 599 ? 90 : 10;\n\n scale.range([m, width - m]);\n\n const svg = d3.create(\"svg\")\n .attr(\"width\", width)\n .attr(\"height\", height);\n\n svg.append(\"g\").call(d3.axisBottom(scale).tickValues(tickValues).tickFormat(d3.format(\",.2f\")));\n\n return svg.node();\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":233,"value":"import {slider} from \"@jashkenas/inputs\"","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"},{"id":1191,"value":"import {colorPicker} from \"@shaunlebron/color-picker\"","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined"}],"files":"$W31","stdlib":"1","titling":"auto"}} 30:"$2f:body"